Both inhibin A and B respond to exogenous follicle-stimulating hormone in the follicular phase of the human menstrual cycle

To ascertain whether changes in the concentrations of the dimeric inhibins A and/or B (INH-A and INH-B) contributed to the previously described dose-dependent increase in immunoreactive inhibin (INH) in response to FSH during the follicular phase of the human menstrual cycle, both dimers were measured by specific two-site assays in stored serum samples from regularly cycling normal volunteers who had received saline as a control (n = 5) or FSH [100 IU (n = 6) or 200 IU (n = 5)] between days 3-5 of the menstrual cycle. Both INH-A and INH-B showed a dose-dependent increase in response to administered FSH; INH-A rose from 13.5 to 35.9 ng/L (P < 0.01), and INH-B rose from 77.8 to 205 ng/L (P < 0.05) at 36 h after 200 IU FSH. Highly significant correlations were observed between INH and each of the specific inhibin dimers (A: r = 0.79, P < 0.001; B: r = 0.76, P < 0.001), and the responses of the two dimers were also highly correlated (r = 0.59, P < 0.001). The response of each inhibin was also highly correlated with the response of serum estradiol (A: r = 0.45, P < 0.001; B: r = 0.40, P < 0.001). When analyzed by ANOVA, the INH response of INR-B was significantly above the control value at 36 h after treatment with both 100 and 200 IU FSH, whereas the response of INH-A was significant only at 200 IU. It is concluded that the concentrations of both dimeric INH-A and INH-B are stimulated by increases in FSH within the physiological range in the follicular phase of the human menstrual cycle and that both contribute to the previously observed rise in INH.